Laser ultrasonics-based material analysis system and method utilizing lamb modes

ABSTRACT

An interferometric system for measuring a property of interest of a target, such as temperature, by detecting the velocities of the symmetric S o ) lamb modes and anti-symmetric (A o ) lamb modes.

This is a divisional of application Ser. No. 08/482,782 filed on Jun. 7,1995, U.S. Pat. No. 5,604,592, which is a divisional of application Ser.No. 08/308,372, filed on Sep. 19, 1994, now abandoned.

FIELD OF THE INVENTION

This invention relates generally to apparatus and method for remotely(non-contact) characterizing one or more properties of a target, such asthe metallurgical status, structural integrity, dimensions, and/ortemperature, through the use of ultrasonic energy that is induced withinthe target and the use of an optical system for detecting acorresponding motion of the surface of the target.

BACKGROUND OF THE INVENTION

The generation of elastic waves in a target is a well-characterizedphenomenon. It is known that when transient changes in the structure ofa target occur, elastic waves are generated both on the surface and inthe bulk of the target. Referring to FIGS. 1a-1d, there are four typesof waves which can propagate in a target, such as a solid 1, namelylongitudinal, shear, Rayleigh or surface, and Lamb waves (shown in FIG.10a). The longitudinal and/or shear waves travel only through the bulkof the solid, with the longitudinal waves having a velocity that isapproximately twice that of the shear waves. The Rayleigh waves travelonly on the surface of the solid with velocities slightly less than thatof the shear waves. The Lamb waves are supported by and propagatethrough very thin solids, and may be used to measure the thickness ofthe solid 1. Longitudinal bulk waves and shear bulk waves have beenextensively used for the detection of flaws, measurements of elasticproperties of solids, and for the monitoring of phase transitions, suchas occurs when a molten metal solidifies. It is also well-known tomeasure the temperature of the solid 1, as the temperature effects thevelocity of the waves within the solid.

A number of different types of transducers have been employed togenerate elastic or ultrasound wave energy in solids. Of most interestherein is the use of a laser (e.g., impulse laser 2) to generateultrasound waves, coupled with the use of a detection laser 3, such asis found in an optical interferometer, to detect a movement of thesurface of the solid 1 in response to the propagating ultrasound waves.

For example, by synchronizing the operation of the interferometer 3 withthe firing of the impulse laser 2, and by determining a differencebetween the impulse laser firing time and the time that the wave isdetected, the velocity of the wave in the solid 1 can be determined; solong as the distance d is known between the spot where the impulse laserbeam 2a impinges and where the detection laser beam 3a impinges. Thedetermined velocity, or `time of flight`, may then be correlated withsome property of interest of the solid, such as the structure of thesolid or the temperature of the solid. For the case where the impulselaser beam 2a and the detection laser beam 3a are directed to oppositesides of the solid, as in FIG. 1d, it is possible to measure thethickness of the solid. The thickness can also be measured, with theimpulse and detection laser beams impinging on the same side, if thesolid is thin enough to support a Lamb wave.

A representative, but not exhaustive, list of U.S. Patents in this andrelated technical areas include the following: U.S. Pat. No. 3,601,490,issued Aug. 24, 1971 to K. Erickson and entitled "Laser Interferometer";U.S. Pat. No. 3,694,088, issued Sep. 26, 1972 to J. Gallagher et al. andentitled "Wavefront Measurement"; and U.S. Pat. No. 4,633,715, issuedJan. 6, 1987 to J. Monchalin and entitled "Laser HeterodyneInterferometric Method and System for Measuring UltrasonicDisplacements".

Also of interest is U.S. Pat. No. 5,286,313, issued Feb. 15, 1994 toThomas J. Schultz, Petros A. Kotidis (an inventor of the subject matterof this patent application), Jaime A. Woodroffe (an inventor of thesubject matter of this patent application), and Peter S. Rostler. Thesubject matter of this U.S. Patent, entitled "Process Control SystemUsing Polarizing Interferometer", is incorporated by reference herein.The preferred embodiment of the system described in this patent employsan XeCl impulse laser in combination with a Helium-Neon-based polarizinginterferometer to provide, by example, remote detection of a temperatureof a workpiece.

One intended operating environment for this type of system is in ametals fabrication and/or treating facility. As can be appreciated, andbecause of the ambient heat, vibration and airborne particulate matterthat is typically found in this type of environment, severe demands andoperating stresses are placed on the interferometer and its associateddetection laser and optical elements.

Another important consideration is the cost of the system, as anindustrial application may require the use of a number of materialsanalysis systems. That is, it is desirable to provide a rugged, compactand low cost system without compromising measurement accuracy andrepeatability.

Although the system described in U.S. Pat. No. 5,286,313 is well-suitedfor use in its intended application, it is an object of this inventionto provide an improved laser ultrasonics materials characterization andanalysis system.

SUMMARY OF THE INVENTION

This invention provides an interferometric-based laser ultrasonicsmaterials analysis system that is improved over known types of systems.This is achieved through a novel combination of laser beam shaping andpointing techniques, the use of a low cost, rugged, and compact diodelaser assembly as a detection laser, various techniques to optimize thesystem for use with the diode laser-based detection laser, and the useof signal processing techniques that compensate for inherentinstabilities and short-term drift in the diode laser. In addition,matched filter processing techniques are disclosed for processinginterferometrically-obtained data points from a target being analyzed.

Also disclosed is a method and apparatus for interferometricallymonitoring a target to determine, in accordance with predeterminedcriterion, the occurrence of a period of time that is optimum forobtaining a data point. In response to detecting such a period animpulse source, such as an impulse laser, is triggered to launch anacoustic wave within the target so that a data point can be obtained. Aplurality of data points so obtained are subsequently processed, such asby the matched filter technique, to determine a property of interest ofthe target. The property of interest may be, by example, the temperatureof the target or the metallurgical status of the target.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention when readin conjunction with the attached Drawings, wherein:

FIGS. 1a-1d generally illustrate various types of waves that may besupported within a solid and various configurations of impulse anddetection lasers for generating and detecting these waves;

FIG. 2 is a block diagram of a laser ultrasonics materials analysissystem in accordance with this invention;

FIG. 3 is an elevational view of one embodiment of a laser ultrasonicssystem constructed in accordance with this invention;

FIG. 4. is top view that illustrates a component placement and layout ofan optical head portion of a system embodiment that is similar to thatof FIG. 3;

FIG. 5 illustrates a transfer function of a polarizing interferometerthat uses two detectors 90° out of phase;

FIG. 6 illustrates exemplary received signals generated by thepolarizing interferometer of FIG. 2, and is useful in describing a`trigger-on-demand` mode of operation;

FIG. 7a is a cross-sectional view of an axicon used for impulse laserbeam shaping in accordance with a first embodiment of this invention;

FIGS. 7b and 7b' are each a cross-sectional view of a waxicon used forimpulse laser beam shaping in accordance with a second embodiment ofthis invention;

FIGS. 7c-7e illustrate various impulse beam shapes, and theirrelationship to a probe beam, in accordance with the invention;

FIG. 8a is a front-facing view of a presently preferred embodiment of abeam-steering mirror that is used for directing an annular impulse beamand a spot probe beam in accordance with an embodiment of thisinvention;

FIG. 8b is a block diagram of an automatic beam steering system inaccordance with an aspect of this invention;

FIG. 9 is a block diagram showing a sub-system of the controller of FIG.2 used for triggering the impulse laser of FIG. 2 at an optimum point onthe transfer function of FIG. 5;

FIG. 10a is a cross-sectional view of a thin solid and illustrates, notto scale, an exemplary Lamb mode being supported by the solid;

FIG. 10b is graph that plots shear mode velocity versus thickness for asolid, and shows the convergence of the dispersive S_(o) and A_(o) Lambmodes into Rayleigh surface waves;

FIG. 10c is a graph that illustrates a difference in propagationvelocity of the S_(o) and the A_(o) Lamb modes;

FIG. 11a is a block diagram showing a first embodiment of a signalprocessing sub-system of the controller of FIG. 2;

FIG. 11b is a block diagram showing a second embodiment of a signalprocessing sub-system of the controller of FIG. 2;

FIG. 12 is a cross-sectional view of a rapid thermal processor (RTP)system that is constructed and operated in accordance with thisinvention;

FIG. 13 is a top view of a furnace showing the system of this inventionbeing used to examine boiler tubes for hot spots;

FIG. 14 is flow chart of a method of this invention;

FIG. 15 is a graph illustrating a phase change that occurs in the makingof steel, and which is detectable by the system of the invention;

FIG. 16 is a simplified cross-sectional view of a laser diode assemblyin accordance with an aspect of this invention; and

FIG. 17 is a flow chart of a method of this invention that employs thedetermination of the TOFs of both longitudinal and shear waves.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a block diagram of a presently preferred embodiment of a laserultrasonics materials analysis system 10 that is constructed andoperated in accordance with this invention. The system 10 generallyoperates by launching an elastic wave within a target, sensing a surfacedisplacement of the target due to the elastic wave, and then correlatingthe sensed displacement with a value of a property or properties ofinterest. As employed herein an elastic wave is intended to alsoencompass an acoustic wave. Also as employed herein a target may be asolid, a semi-solid, or a liquid.

A system controller 12, such as an embedded microprocessor or anexternally connected computer or workstation, includes a user interface12a that includes, by example, a touchscreen and/or a conventionalkeyboard and/or a pointing device (e.g., mouse) in combination with agraphical display device through which a user is enabled to interact anddirect the operation of the system 10. An impulse laser 14 is controlledby the controller 14 to provide an impulse beam 14a to the surface of anobject, hereafter referred to as a target 16 (the target forms no partof the system 10, and is shown only for completeness). The impulse beam14a causes a localized heating of the target 16 and launches an elasticwave within the target as described previously. A displacement of thesurface of the target 16 due to the elastic wave is detected by apolarizing interferometer 18 that is constructed and operated inaccordance with this invention.

The interferometer 18 includes a detection laser 20 which, in thepresently preferred embodiment of the polarizing interferometer 18,includes a diode laser 22. The diode laser 22, provides high power (>100mW) along with the combined characteristics of small dimensions, lowcost (relative to more conventional detection lasers such as theHe--Ne), and a long coherence length (i.e., narrow bandwidth).Commercially available diode laser systems can provide >100 mW of powerwith bandwidths that range from 10 MHz to less than 10 KHz. Theselection of a particular diode laser for use in the system 10 is afunction of the required power, coherence length, wavelength, systemcompactness, and cost. Included with the detection laser 20 is aconventional Faraday rotator (not shown) to prevent any reflected laserlight that returns from the target 16 from effecting the performance ofthe diode laser 22. Although the operating characteristics of theinterferometer 18 ensure that the returned laser light intensity is verysmall, even a small amount of returned laser light can adversely effectthe wavelength characteristics of the laser diode 22. The output of thedetection laser 20 assembly is a source beam 20a.

Reference is now made to FIG. 16 for illustrating a novel and preferreddiode laser assembly 21. It is first pointed out that some diode lasershave been conventionally stabilized by an external cavity formed by agrating mounted in a Littrow configuration. The first order reflectionoff the grating provides feedback into the laser cavity and the zeroorder reflection provides the output coupling.

The diode laser assembly 21 of FIG. 16 employs a novel technique formounting and aligning a grating which is very stable (thermally andmechanically), and at the same time inexpensive to manufacture andsimple to construct.

A laser diode 21a and a collimating lens 21b are mounted on an end of asupport structure (e.g., a tube 21c made from stainless steel or someother suitable rigid material). By example, the tube 21c has a length ofsix inches and a diameter of two inches. The diode laser 21a has ananti-reflection (AR) coating on its output facet to eliminate facetmodes which would conflict with external modes established by a grating21d. The grating 21d is mounted on a fixed angle pedestal 21e, the angle(θ₁) of which is selected to provide a desired operating wavelength ofthe laser diode 21a. By example, if a 1800 groove per millimeter gratingis used, the accuracy of the machined angle required for tuning thelaser diode emission to ±1 nm is approximately ±0.8 degrees, a valuewell within simple machining tolerances.

Since the tuning range of the diode laser 21a is large (10's of nm) theangle of the tuning axis (in the plane of the drawing) is not criticaland, thus, no adjustment mechanism is necessary (although one may beprovided if desired). However, the angle out of plane (the alignmentaxis AA) cannot be readily machined to sufficient accuracy. As a result,this angle is adjusted by rotating a wedge 21f about the axisapproximately parallel to the laser beam. The wedge 21f is positionedbetween the diode laser 21a and the grating 21d and functions to changethe angle of the laser beam. The wedge 21f is supported by a rotationstage mount 21g. In FIG. 16 the angle θ₂ is the Littrow angle associatedwith the grating 21d, and the optical cavity external to the diode laser21a extends generally to the face of the grating 21d.

If the wedge 21f is initially rotated so that the deflection produced isin the same plane as the incidence plane on the grating 21d, then smallrotation angle changes of the wedge 21f will have very little impact onthe tuning angle, while allowing sufficient control on the alignmentaxis angle. As an example, if a 10 degree quartz wedge is used, a 10degree rotation of the wedge 21f will result in a 0.8 degree change inthe alignment axis, with only a 0.9 nm change in the tuned wavelength.

The wedge 21f is preferably anti-reflection coated to minimize cavitylosses, and is also tilted with respect to the beam axis so thatreflections off either wedge surface will not go back into the laserdiode 21a.

Referring again to FIG. 2, the source beam 20a is provided to a beamexpander 24. The characteristics of the beam expander 24 directly impactthe light collecting power of the sensor system described below. Ingeneral, the larger the beam diameter, the larger the return specklesize, and thus the greater is the fraction of the total returned powerthat is available to interference signal generation.

The beam expander 24 can be placed either before or after a polarizingbeam splitter 28 that is described below. Placing the beam expander 24after the beam splitter 28 has the advantage that the interferometeroptics can be made smaller and also independent of the expansion ratioof the beam expander 24. However, placing the beam expander 24 at thisposition requires that the detection or probe beam (PB) 28b pass throughthe beam expander 24 twice. As a result, the quality (cost) andalignment of the beam expander 24 becomes important to the overalloperation of the system 10. As such, and although it is preferred toplace the beam expander 24 before the polarizing beam splitter 28, asillustrated in FIG. 2, the teaching of this invention is not so limited.

The expanded source beam 24a next encounters a halfwave plate 26 that islocated before the polarizing beam splitter 28. The halfwave plate 26provides a mechanism for setting a desired ratio for a reference beam(RB 28a) to probe beam (PB 28b) intensity. Varying the rotation angle ofthe halfwave plate 26 rotates the polarization of the laser beam and, incombination with the operation of the polarizing beam splitter 28 thatis described next, thereby controls the fraction of the beam going intothe PB 28b and into the RB 28a of the interferometer 18.

In accordance with an aspect of this invention the halfwave plate 26 maybe coupled to a mechanism, such as a motor, for imparting a rotarymotion to the halfwave plate 26. In this embodiment the controller 12automatically monitors the signal returned from the target 16 andcontrollably rotates the halfwave plate 26, via signal line 12b, so asto optimize the relative intensities of the reference and probe beams.Alternately, this function can be performed by user who monitors agraphical display provided by a camera 44 (described below).

The rotated beam 26a that passes through the halfwave plate 26 is splitinto the RB 28a and PB 28b by the polarizing beam splitter 28, with theRB 28a and PB 28b having relative intensities set by the rotationimparted by the halfwave plate 26. After the reference and probe beams28a and 28b leave the polarizing beam splitter 28 each passes through anassociated 1/4 wave retardation plate 30a and 30b, respectively. Plates30a and 30b are aligned so that both of the RB 28a and PB 28b arecircularly polarized.

The path length of the RB 28a is adjusted to reduce the noise in thesignal that is detected from a combined beam (CB) 28c. The degree towhich the lengths of the probe leg and the reference leg are matched isa function of the bandwidth of the diode laser 22, the fraction of thesignal noise that is attributed to any frequency jitter of the diodelaser 22, and the impact of the length of the reference leg on theoverall compactness of the interferometer 18.

Included within the RB 28a leg are a plurality of folding mirrors 32aand 32b and a corner cube reflector 34. It is important to the operationof the interferometer 18 that the reference leg return beam be at thesame angle (opposite direction) as the outgoing reference beam. Thisimportant goal is achieved in a simple, compact, and inexpensive mannerusing the corner cube 34. In contrast, a simple mirror would requirecareful and precise adjustment, and very high quality mounts all alongthe reference path to maintain the alignment. In the presently preferredembodiment of this invention these requirements are eliminated by theuse of the corner cube 34 (preferably gold coated and hollow) whichterminates the reference beam path leg while preserving the polarizationcharacteristics of the RB 28a.

The PB 28b is focussed to a point on the target 16 using a lens 36 thathas a focal length equal to the distance to the target 16. That portionof the PB 28b that reflects from the surface of the target 16 issubsequently collimated by the lens 36 as it travels back into theinterferometer 18.

The same prism (the polarizing beam splitter 28) that is used to splitthe polarized beam 26a into the RB 28a and the PB 28b is also is used torecombine the RB 28a and the returned (reflected) portion of the PB 28binto a combined beam (CB) 28c. Because the RB 28a and the PB 28b arecircularly polarized, and must pass back through the 1/4 wave plates 30aand 30b, respectively, they are again linearly polarized, but at theopposite orientation than their original linear polarizations. Becauseof this, the CB 28c does not go back toward the diode laser 22, butinstead is directed into a signal detection portion of theinterferometer 18.

As was stated, after the RB 28a and the PB 28b are combined they areboth linearly polarized, but of the opposite sense. In order to generatean interference signal (detectable interference fringes), a polarizingprism or beam splitter 38 oriented at 45° is used to select a projectionof the polarization axis of each of the RB 28a and the PB 28b along acommon axis. This results in two combined beams whose interferencesignal is 180° out of phase. Either or both of these beams can be usedto provide the signal necessary for process analysis. For example, FIG.5 illustrates the use of two photodetectors (PD-1 and PD-2) fordetecting two combined beams. A combined beam focussing lens 40 is usedto focus the CB(s) 28c onto a radiation sensitive surface of one or morephotodetectors 42. The lens 38, in combination with an aperture 41, canalso be used to spatially block out light other than that of thecombined beams from impinging on the photodetector 42.

The photodetector 42 may be provided in a number of suitable forms,depending on performance characteristics.

Both conventional pliotomultiplier tubes and hybridphotodiode/amplifiers are suitable embodiments for detecting the lightand dark pattern that results from the interference of the RB 28a andthe returned portion of the PB 28b within the CB 28c. The output of thedetector 42 is provided to the controller 12 for signal processing inthe manner described below.

An optional camera 44 is primarily used as a diagnostic tool, i.e., thecamera 44 useful for optimizing the detected signal when an operator issetting up and controlling the system 10. For example the camera 44,which may be a conventional CCD device that provides an output to adisplay monitor of the user interface 12a, provides visual feedback tothe operator for best signal return, which implies a best pointing angleto the target 16. However, in one embodiment of the invention anautomatic beam steering system performs this function without operatorintervention. The camera 44 may also be used for alignment of the signaland reference beams. The camera 44 can also be employed to determine therelative intensity of the reference and signal beams and, based on theindicated intensities, the operator is enabled to rotate the halfwaveplate 26 to achieve an optimum intensity distribution for optimum fringecontrast.

It should be realized that if the camera 44 is eliminated a secondphotodiode 42 can be installed in its place. The use of a secondphotodiode 42 enables a square and add signal processing technique to beused as described in U.S. Pat. No. 5,286,313, which has beenincorporated by reference herein.

FIG. 3 depicts an elevational view of the system 10, and shows abulkhead-mounted optical head 11, a base unit 13, and the controller 12.The optical head 11 is constructed to have a slide-in unit 11a thatcontains the impulse laser 14, the interferometer 18, and all associatedelectronics and controls. Preferably, the optical head 11 includes ajacket 11b through which a cooling fluid (e.g., water) is flowed duringthe operation of the system 10. The slide-in unit 11a is thermallycoupled to the water jacket 11b when inserted into the head 11, whichthereby functions to remove the heat that is generated by the operationof the lasers and associated electronics, although a primary function ofthe water jacket 11b is to remove the external heat load during hightemperature operation. The base unit 13 contains all required powersupplies and provides, via cabling 13a and 13b, an interface to thecontroller 12. In a further (unillustrated) embodiment of the inventionthe optical head is instead tripod-mounted and may not be water-cooled.

FIG. 4 is a top view of an exemplary embodiment of a slide-in unit 11athat is suitable for use in the above-mentioned tripod-mountedembodiment of the invention. A baseplate 50 provides a rigid support formounting all of the required lasers and optical components. The impulselaser 14 has its output connected to a variable beam attenuator 50 andto a beam-shaping negative axicon 52a (FIG. 7a). The negative axicon 52aprovides, in accordance with an aspect of this invention, a ring-shapedannular impulse beam 14a for impinging on the target 16. In anotherembodiment of the invention a waxicon 52b (FIGS. 7b and 7b') is employedinstead. One difference between these embodiments is that the negativeaxicon 52a works in transmission, while the waxicon 52b works inreflection. A second difference is that the use of the waxicon shortensthe beam path.

The shaped impulse beam is folded by mirrors 54, 56, 58 and combiner 60before being provided to rotatably mounted beam steering mirrors 62 and64, connected to a motor 64a, which direct the impulse beam 14a towardsthe target 16 at a controlled and optimum angle. This aspect of theinvention is described in detail below with regard to FIGS. 8a and 8b.

The detection laser 20 is provided as a modular unit that includes thelaser diode 22 and an externally mounted resonant cavity for narrowingthe bandwidth of the laser diode 22. It is also preferred to use anantireflection (AR) coating to eliminate any internal modes of the laserdiode. It is further preferred to employ a current control techniquethat uses a photodiode detector to sense any instability in the outputof the laser diode 22 and, in response, that perturbs the laser diodecurrent by some predetermined amount (e.g., 0.1%-1%). The laser diode 22is operated as a continuous duty device. That is, and as will be madeapparent below, an interference signal from the target 16 is availablewhether or not the impulse laser 14 is being triggered. A preferredembodiment of a diode laser assembly is illustrated in FIG. 16 and wasdescribed previously.

The output of the laser diode 22 is provided via mirrors 66 and 68 tothe aforementioned Faraday rotator 70 and then to the halfwave plate 26(beam polarization rotator). In this embodiment of the invention thebeam polarization rotator 26 is positioned in front of the beam expandertelescope 24. Coupled to the output of the beam expander telescope 24 isthe polarizing beam splitter 28. The output of the beam splitter 28impinges on a mirror 72 from which it is directed to an optical pathwherein the beam is folded a number of times between a mirror 74 andreflectors 76 and 78. The probe beam 28b is output along with theimpulse beam 14a towards the target 16 via the beam steering mirrors 62and 64.

The folding of the probe beam between mirrors 74, 76 and 78 is an aspectof this invention related to the use of the diode laser as the detectionlaser 20. The coherence length of a typical diode laser is in the rangeof 30 meters to 40 meters. For an unequal path length interferometer(wherein a difference between the probe beam path length and thereference beam path length is large), this would imply that suitableinterferometric detection would occur. However, there still may besufficient frequency jitter to contaminate the measurement of smalldisplacements of interest. This is overcome in the system 10 by pathmatching the probe beam path to the reference beam path with the mirrors74, 76 and 78 to provide an equal or approximately equal path lengthinterferometer and a significant increase in the SNR. This path matchingis used in combination with the grating 21d (FIG. 16), the Faradayrotator, the beam expanding telescope 24, and the focussing lens 36 (toprovide at least one bright speckle), in order to optimize theinterferometer 18 for use with a diode laser as the detection laser.

Referring again to FIG. 4 the components 80 and 82 are shutters. Alsoshown is the camera 44, the photodetector 42 and, in dashed outline 84,a support electronics package. As can be appreciated, the impulse lasersystem and the interferometer 18 are provided in a small area and thebeams are tightly folded to minimize the required mounting area.

It is within the scope of the invention to provide the impulse beam 14awith a shape that corresponds to a ring, a line, or a point. Thering-shape is preferred (using the negative axicon 52a or the waxicon52b), but there are many applications where the other shapes becomeimportant. The probe beam 28b is preferably tightly focussed to adiffraction limited spot. For the annular ring-shape of the impulse beam14a the probe beam 28b is located at the center of the ring (FIG. 7c),and the time of flight (TOF) of the elastic wave launched by the IB 14ais measured across the radius of the ring. For the line and pointimpulse beam shapes the probe beam 28b is separated by a fixed distanced, for example 0.5" to 1", away from the impulse beam 14a (see FIGS. 7dand 7e, respectively), and the TOF is measured across the distance d.

An important aspect of the design of the negative axicon 52a used inthis invention is that the conical surface is concave. The same impulselaser ring-shape on the target can be obtained with a convex surface(positive) axicon, but immediately after the axicon there will be acaustic focus which creates a line-shaped zone of intense laser fluencewhich can cause damage to optics placed in its path. The length of thiszone extending from the axicon is a function of the angle of the axiconand the diameter of the impulse beam, and can extend several inches.With the concave surface axicon there is no limit on how closesubsequent optics can be placed. As such, the use of the concave surfaceaxicon is preferred, although the teaching of this invention is notlimited to only this configuration.

A problem that arises during the use of an axicon is that the impulsebeam 14a ring diverges and the diameter of the ring on the target willvary as a function of distance to the target. Such a situation isunacceptable as the system 10 would not operate correctly with moving orvibrating targets because the target distance, and thus the diameter ofthe IB 14a ring, will change continuously. In this case the time ofarrival of the elastic waves at the location of the PB 28b will also becontinually changing, and there would be no way to distinguish between achange in the arrival time due to the target motion (a change in ringdiameter) or that due to a change in a property of the target (a changein elastic wave arrival time).

This problem is overcome in the system 10 by ensuring that thering-shaped IB 14a exiting the head 11 is of constant diameter and isnot expanding (diverging) or converging. This is achieved by causing theimpulse beam 14a to travel a predetermined distance within the head 11before it is focused onto the target 16. The predetermined distance isapproximately equal to the distance of the head 11 to the target 16, andis achieved through the use of adjustable path length folding opticsrepresented by the components 54, 56, 58, 60 and 62 of FIG. 4.

The optical properties of the reflective waxicon 52b of FIGS. 7b and 7b'also solve this problem of impulse beam divergence or convergence andprovide an impulse beam ring of constant diameter, while reducing oreliminating the requirement to provide the extended light path insidethe head 11. As a result, the use of the waxicon 52b is desirable toreduce the volume of the head 11.

Referring now to FIGS. 7b and 7b' there is illustrated one suitableembodiment of the waxicon 52b. The waxicon 52b is comprised of asubstrate 53, such as aluminum, having a diamond machined reflectivesurface 53b (λ/4). A suitable diameter is three inches, and a suitablemaximum thickness (TH) is one inch. A suitable value for angle θ₁ is45°, while a suitable value for angle θ₂ is approximately 45.3°. Inresponse to an input beam 53c having a diameter of approximately 18 mman annular output beam 53d is generated with a diameter (BD) ofapproximately two inches. Other materials, dimensions and angles may beemployed, depending on the requirements of a particular application.

As has been previously indicated, the impulse beam 14a and the probebeam 28b may be located on the same side of the target 16 or, withadditional reflectors and path optics, on opposite sides of the target16.

The stability of the probe beam 28b is determined by the duration of atime interval during which the wavelength of the detection laser 20 canshift by no more than a predetermined maximum quantity or number of wavenumbers. In the preferred embodiment of the invention the maximumquantity or number of wave numbers corresponds to a frequency in therange of 10 kHz to 100 kHz. However, this stability requirement is at orbeyond the limit of the capabilities of currently available laser diodesystems, or of most if not all types of laser systems that would berequired to operate in the adverse environments within which the system10 may be required to operate.

What makes the measurement of the minuscule target surface displacementspossible is the fact that the detection beam is required to be stablefor only a short period of time, e.g., 10 μsec at most, whichcorresponds to the time required to fire the impulse laser 14 and obtaina reading from the photodetector 42. In other words, this short termstability feature would be of little or no interest to a conventionaluser of diode lasers, but is exploited to its maximum potential in thesystem 10 if this invention.

In order to accommodate the different wavelengths of the diode laser 22and the impulse laser 14, and still maintain the compactness of the head11, a combination of mirrors 62 and 64 is employed to steer both beams(it being remembered that the impulse beam 14a may be a ring with the PB28b at its center).

As is shown in FIG. 8a, preferably the beam steering mirrors (actuallythe combination of the mirrors 62 and 64) have a reflective "patch" intheir center, i.e., a small area 64a that is coated with a metal (e.g.,gold) to reflect the PB 28b (which is in the infrared spectrum), whilethe remainder of the surface of the mirror 64 has a conventionaldielectric coating 64b to reflect the impulse beam 14a, which may be inthe green portion of the visible spectrum. Since the energy flux of theimpulse beam 14a is quite high, providing a dielectric coating isdesirable to avoid optical damage. In addition, the PB 28b must maintainits polarization during reflections, and hence the metallic (gold) patch64a is preferred over a conventional dielectric-coated mirror, whichwill induce a change in polarization. The use of specific reflectionmaterials and dielectric coatings can be generalized to whatevercombination is appropriate for the wavelengths that are being used.

In this regard the choice of the IB 14a and the PB 28b wavelengths isnot random, and there are several criterion involved in a specificchoice of wavelengths. For example, the laser light interaction with thetarget surface is wavelength-dependent, and it has been found that metaltargets respond best to green or ultraviolet (UV) wavelengths for the IB14a. However, many composite materials give optimum results with IB 14awavelengths in the far infrared. In addition, the sensitivity of thephotodetector 42 is a function of the wavelength of the CB 28c and,hence, the emission wavelength of the laser diode 22.

While the system 10 is observing a target with a very diffusereflectivity (near Lambertian), it is not critical that the viewing axisis normal to the surface. In fact, the intensity is still 50% of itspeak value when the viewing axis is 45 degrees off normal. There are,however, surfaces such as those associated with non-polished metallicmaterials where the intensity drops of much faster as a function ofangle. The intensity may drop to 10% of its peak value in a range of 1to 10 degrees. For these situations it is desirable to provide anautomatic system which adjusts the beam angle (preferably both impulseand probe) to stay near normal to the surface. This is important forapplications such as viewing a moving strip in a metals processing plantwhere a common twisting motion of the strip can change the angle by asmuch as ±10 degrees.

Referring to FIG. 8b this is accomplished by providing an alignmentlaser 65a, such as a diode laser operating at 670 nm (i.e. differentwavelength from the wavelength (e.g., 830 nm) of the interferometerprobe laser 20). The output of the alignment laser 65a passes through alens 65b which is selected so that the beam coming out of the instrumentilluminates a spot on the target which, relative to the position of theinstrument window, subtends an angle larger than the possible angularvariation of the target 16. The alignment beam from alignment laser 65ais combined with the PB 28b from detection laser 22 using a dichroicbeam splitter 65d. The dichroic beam splitter 65d reflects the alignmentlaser beam AB but transmits the PB 28b. Alignment laser light returningfrom the target 16 is imaged onto a position sensitive detector 65e withan imaging lens 65f. One suitable embodiment for the detector 65e is awell known silicon quadrature detector. The imaging lens 65f is selectedto have the beam spot on the target imaged to about the same size as thedetector 65e. A 50/50 beam splitter 65c provides a simple method to bothreflect out the alignment laser, and pass the returning beam (returningon the same axis AB) to the imaging lens 64f and the detector 65e.Although half the intensity is lost on the outgoing path and half on theincoming path, this technique is simpler and less expensive than using apolarizing beam splitter and quarter wave plate to eliminate theselosses. However, target reflectivities are low, or available alignmentlaser powers are too low, this more involved technique can be used.

If the target 16 is very diffuse then the intensity distribution on thedetector 65e is very similar to that of the alignment laser-producedspot on the target 16. If the reflectivity drops of quickly with anglethere will be a bright spot within the image on the detector 65e. If theviewing angle is normal to the surface of the target 16, this brightspot will be centered within the image on the detector 65e. Theintensity information from the detector 65e is provided to closed loopcontrol circuitry (not shown) which drives xy tilt mechanisms 65g onsteering mirror 64 to bring the bright spot to the center of the imageon the detector 65e. By subsequently employing the xy tilt mechanisms65g to maintain the spot at the center of the image received by detector65e, the probe and impulse beams 28b and 28a are maintained normal tothe surface of the target 16, which is the desired result.

Some materials, such as rolled aluminum, have different reflectivityangular distributions for the axis along a strip relative to the axisperpendicular to the strip. This may necessitate greater sensitivityalong one axis compared to another. This can be accomplished by using anelliptical spot on the target 16 (minor axis in the direction of greaterrequired sensitivity) and providing the lens 65f as one or morecylindrical lenses to image the orthogonal axis of the ellipseseparately on to the detector 65e. The cylindrical lenses function tocreate a circular spot on the detector 65e.

The transfer function of a two detector polarizing interferometer 18 isshown in FIG. 5. For a given target 16 surface displacement (x-axis),the interferometer 18 generates a signal (y-axis) given by theillustrated sinusoidal curves. FIG. 5 shows the transfer functions fortwo photodetectors 42 that are 90° apart in phase, although it should berealized the system 10 may operate with one, two, or even morephotodetectors 42. What should be evident from this transfer function isthat the most sensitive, and hence the most desirable and optimum pointsof operation are at the maximum slopes, i.e., points E, C, F, D.

An interferometer may use vibrating mirrors or other moving parts toadjust or dither the reference leg of the interferometer so as to forcethe system to operate at such points. However, this approach may not bedesirable for all applications in that it adds to the overall complexityand cost of the system.

In accordance with a further aspect of this invention the system 10operates at these optimum points, without the use of moving opticalcomponents, by an algorithmic technique that results in the triggeringof the impulse laser 14 only at times when the signal received from thephotodetector(s) 42 is optimum.

FIG. 6 shows a typical signal received from a target 16 (moving in thiscase, but static targets have a similar response). As shown, there are"bursts" of sinusoidal signals designated as A, D, and C, interspersedwith regions B of little or no sensitivity (flat line). The burstsoccur, typically, over a time (t) of two to four milliseconds.

The regions A, B and C represent `poor` signal regions, while the regionD represents a `good` signal region, i.e., low frequency target motionand a high signal amplitude.

The poor region A exhibits a high signal amplitude, but also has a highfrequency of target motion. The existence of the region A is usually dueto target motion and, in most cases, regular environmental vibrationsthat cause oscillations of low frequency. It is important to note thatthese oscillations are not of the same frequency as the environmentalvibrations, but instead represent the number of wavelengths per unittime received by the interferometer 18. For example, a low frequency,but very large amplitude, external vibration can cause the target 16 tomove by many millions of wavelengths in a very short time. On the otherhand, the same condition can be caused by a high frequency, but lowamplitude vibration. If the frequency of these wavelengths reaches theuseful data range (for example, 300 kHz to 2 MHz), then no subsequentfiltering action can separate the desired signal from the externalvibrations. The situation may be referred to as "frequency poisoning".

The existence of the regions B of FIG. 6 can be attributed to one ormore of the following: (a) no return light, hence no interference, whichcan be caused by receiving a dark speckle from the target 16 (light anddark speckles are randomly distributed); and (b) no interference due toa lack of diode laser 22 coherence, which can be caused by instabilityin the laser operation or signal/reference leg path differences beinglarger than the coherence length of the diode laser 22.

The region C exhibits a low frequency of target motion, but also has alow signal amplitude. This can be due to a lack of contrast in theinterference fringes or by an insufficient amount of returned light.

In the regions A, B and C of FIG. 6 any obtained data would not beacceptable, and it is thus desirable to inhibit the taking of data withthe interferometer 18 during these times.

In accordance with this aspect of the invention the system 10 operateswith an impulse laser 14 "trigger on demand" (TOD) technique thattriggers the impulse laser 14, and hence collects data, only during aperiod of reception of a "good" signal (i.e., region D of FIG. 6).

Referring to FIG. 9, the controller 12 includes a bandpass filter 90that is coupled to the photodiode (PD) 42. When operating in theRayleigh mode the desirable signals are generally in the range ofapproximately 1-5 MHz, while when operating in the Lamb mode thedesirable signals are generally in the range of approximately 100 kHz to1 MHz. The output of the filter 90 is connected to an envelope detector92 and to a zero crossing detector 94. The envelope detector 92determines the amplitude of the bursts received from the photodiode 42,while the zero crossing detector 94 detects the zero crossings and,hence, the frequency component of the sinusoidal bursts. An analyzer 96monitors the output of the envelope detector 92 and the output of thezero crossing detector 94 to determine a condition wherein both theburst amplitude and frequency indicate that a `good` burst is occurring.The occurrence of such a good burst indicates a lack of all or most ofthe undesirable conditions described in conjunction with the regions A,B and C or FIG. 6 (e.g., loss of laser diode coherence, unacceptabletarget motion, the reception of a dark speckle, etc.). The analyzer 96then triggers the impulse laser 14 to initiate a measurement during theremaining period of the good burst. This trigger point (TP) is indicatedin the region D of FIG. 6.

Not only is the trigger point initiated during the good burst, but thetiming of the trigger may also be selected so that an expected elasticwave detection will occur at or near an optimum point on the sinusoidalsignal.

By example, and referring also to FIG. 5, if the expected TOF isapproximately 1/4 of the period of the sinusoidal signal then the TPoccurs at a point corresponding to point A in FIG. 5, which results inthe launched elastic wave passing under the probe beam 28b at or aboutthe optimum measurement point C on the sinusoid. Of course, anylatencies in the operation of the impulse laser 14 are also considered,and the initiation of the trigger 96a is adjusted accordingly. Typicallatencies are in the range of 100 μsec to 200 μsec.

It should be realized that the functions depicted in FIG. 9 may beexecuted wholly or partially by a suitably programmed processing device.

Thus, and referring to FIG. 14, in accordance with a method of thisinvention there are performed the steps of (A) detecting the envelope ofthe received signal to determine the amplitude of the sinusoidaloscillations; (B) detecting zero crossing points of the sinusoidaloscillations to identify the frequency of the burst; (C) analyzing thedetected amplitude and the detected frequency to determine an occurrenceof a good burst; and (D) triggering the impulse laser 14 at anappropriate time in order to collect data at the maximum slope operatingpoints of the polarizing interferometer 18, i.e., points E, C, F, or Dof FIG. 5.

Irrespective of how the impulse laser is triggered (i.e., steps A-D arepreferred but not required), the signal processing of the receivedsignal proceeds as follows: (E) acquiring a trace of signal vs. time;(F) accepting or rejecting the data based on criteria of allowablesignal noise; (G) squaring the data; (H) accumulating similar datatraces and combining them by adding the data, or by using techniquessuch as weighted averaging; (I) using a `matched filter` to extract afeature of interest from the processed signal; (J) calculating the TOFfrom the `matched filter`; and (K) converting the TOF to usefulproperties of the material (such as temperature, metallurgical status,etc.) by using predetermined calibration curves.

In this regard reference is now made to FIGS. 10a-10c and 11. Theaforementioned Rayleigh waves are surface waves associated withrelatively thick targets. In contradistinction, the Lamb waves aresupported only in relatively thin targets (e.g., d=up to approximately0.1"). As shown in FIG. 10b the Lamb waves are characterized by asymmetric mode (S_(o)) and an asymmetric or anti-symmetric mode (A_(o)).The A_(o) mode is dispersive. The Lamb modes propagate through thetarget in a waveguide-like fashion. The S_(o) mode is typically moredifficult to observe because it is relatively weak compared to the A_(o)mode. As shown in FIG. 10c, these two modes travel with differentvelocities within the target. As the thickness of the target increasesthere is a smaller difference between the velocities until the Rayleighregime (i.e., non-dispersive surface wave mode) is entered.

In accordance with an aspect of this invention the S_(o) and A_(o) Lambmodes are detected and are employed in combination to obtain thetemperature of the target 1. That is, equations with two unknowns(thickness and temperature) are solved based on the S_(o) and A_(o)velocities which are detected in response to the application of theimpulse beam 14a. One particularly useful application is in detectingthe temperature of a thin substrate, for example a silicon substratehaving a thickness in the range of approximately 200 to 400 micrometers.An embodiment of this application will be described below with referenceto FIG. 12.

Reference is now made to FIG. 11a which shows a portion of thecontroller 12 in accordance with one embodiment of this invention. Inthis embodiment a matched filter library 100 is employed in conjunctionwith a matched filter processor 102 to determine the thickness of asubstrate. As an example, for the Lamb mode case a plurality of wavetemplates or shapes 100a are digitized and stored during a training modeof operation using samples of predetermined thickness (100b). The storedshapes are each based on a plurality of measurements taken on a singlesample. The measurements are preferably filtered and otherwise processedto eliminate noise and other artifacts of the measurement process. Eachstored wave shape 100a thus represents an average of the plurality ofmeasurements. The training mode is indicated schematically by the switchS1 being in a closed position. During the normal mode of operation thePD 42 output signal is applied to the matched filter processor 102 as aninput signal. Individual ones of the stored shapes 100a are compared tothe input signal to determine a best match through an autocorrelationtechnique (indicated by the arrow). The best match shape 100a is thencorrelated with its associated thickness 100b. Having determined thethickness of the sample, the temperature can be determined.

While a plurality of stored waveshapes 100a are preferred when operatingin the Lamb regime, only a single filter shape is normally required whenoperating in the Rayleigh regime. Alternately, when operating in theRayleigh regime a peak detection technique can be employed, as describedin U.S. Pat. No. 5,286,313.

Reference is now made to FIG. 11b which illustrates a second signalprocessing embodiment, specifically a calibration technique for velocitymeasurements. In general, this is a method to convert the TOFmeasurements to velocity and then to, by example, temperature. Thistechnique is especially applicable to thin targets, where Lamb modes areused and dispersive behavior is observed. Although temperature is usedas an example, the method can be generalized for other materialproperties. Furthermore, although the TOF of the elastic wave isillustrated, this embodiment may be generalized to determine a timevarying characteristic (e.g., frequency, phase, etc.) of the elasticwave.

The inputs to this signal processing embodiment may be manually orautomatically generated. For example, the thickness of the target can beautomatically inferred by the method described in FIG. 11a, or may bemanually inserted from the user interface 12a.

The inputs are the target thickness 104 and material type 105. Thematerial type 105 may be, by example, the identification of an alloy.For a single sided configuration (see, for example, FIGS. 1a and 1b),the separation distance (SD) between the impulse beam 14a and the probebeam 28b may be provided as a constant. Already stored in the processorare the following: a library of "matched filters" 106 and a library ofcalibration curves 107. The library of calibration curves 107 mayrepresent, by example, velocity vs. temperature (see, for example, FIG.15) or, more generally, one or more ultrasonic pulse or elastic wavecharacteristics vs. material property (e.g., temperature). The thicknessand material type inputs 104 and 105, respectively, are used to selectone of the stored matched filters from library 106 and a calibrationcurve from the library 107. Having selected a corresponding matchedfilter from the library 106, the processed data waveform is thencompared with the selected matched filter until a best match isobtained. This is a correlation operation that yields the TOF 106a. TheTOF 106a is then converted to the velocity of the elastic wave based onthe SD.

Then, using the calibration curve selected from library 107 as afunction of the thickness 104 and material type 105 inputs, thetemperature or some other material property is determined in block 108and is output as a result 109.

An application of the Lamb mode processing technique of this inventionis illustrated in FIG. 12. This figure generally shows a rapid thermalprocessing (RTP) system 110 which is used for the thermal processing ofa silicon wafer 112. A heater module 114 surrounds a portion of aprocess chamber 116 which is covered by a layer of thermal insulation118. A lower transfer chamber 120 enables the wafer 112 to be loadedinto and extracted from the process chamber 116. After being loaded anelevator assembly 122 is used to raise and lower the wafer 112 withinthe process chamber 116. A very rapid heating (e.g., 50-100°/sec) of thewafer 112 occurs during this process. Also shown in FIG. 12 are variousother system components such as a process controller 124, an elevationmotion controller 126, and an associated motor/encoder 128 and amplifier130 for raising and lowering the elevator assembly 122. A pyrometer head132 is connected via an optical fiber 134 to a pyrometer 136 formeasuring, by emissivity, the temperature within the process chamber116. The pyrometer 136 is interfaced to the process controller 124 toclose the temperature control loop.

In accordance with the invention the RTP system further includes thelaser ultrasonics materials analysis system 10 of this invention. Thehead 11 is disposed so as to direct the impulse beam 14a and the probebeam 28b onto a surface of the wafer 112 during the thermal processingof the wafer. Because of the thinness of the silicon wafer 112 the Lambmode of operation (FIG. 11b) is preferably utilized to determine thewafer temperature. Due to the crystalline nature of the silicon wafer112 the S_(o) Lamb mode is more apparent because there is lessbackground noise than would be found in, for example, a metal substrateof the same dimensions. This enables temperature measurements of thewafer 112 with an accuracy equal to or less than ±5° F. This temperaturemeasurement occurs without using the emissivity of the silicon wafer,which would be difficult to accurately measure within the high ambienttemperature of the process chamber 116.

This is an important application of the invention as it avoidsprocessing errors which can result in the destruction of the wafer 112.As larger wafers come into use (e.g., 8" to 12" wafers) the expense of asingle wafer, which may have a very significant production costassociated therewith, makes an accurate measurement of wafer temperaturevery important.

FIG. 13 shows another exemplary application of the system 10. In FIG. 13the system 10 is used to monitor the surface temperature of boiler tubes140 within a furnace 142. Such an arrangement is often found within apetrochemical refinery. One or more view ports 144 enable optical accessto the boiler tubes 140. As shown in the enlarged detail, the IB 14a andPB 28b are directed onto the surface of a boiler tube wall 140a. If adeposit build up (shown generally as 140b) occurs on the inner surfaceof the boiler tube wall 140a, a hot spot is created on the outersurface. The presence of such a hot spot is undesirable in that it caneventually lead to metal fatigue and the failure of the affected boilertube 140. Since the boiler tubes normally carry a pressurized heatedvolatile fluid, such a failure can be catastrophic.

The use of the system 10 enables a remote (non-contact) and veryaccurate reading of surface temperature along the boiler tube outerwalls and thus enables a detection of surface hot spots before metalfailure occurs.

An exemplary, but certainly not exhaustive list of other applicationsthat may benefit from the remote, contactless temperature measurementsystem of this invention include the heat treating industry (measurementof internal and surface temperature of steel, aluminum, etc.); thecarburizing industry (measurement and control of carburization depth andsurface hardness); phase change processing (as shown in FIG. 15, thedetection of onset and completion of phase changes during steelprocessing); and nondestructive inspection (detection of internal flaws,cracks, and voids in structures). In the flaw detection applicationadditional signal peaks are observed which arise from internal flawswithin a structure (such as bridge support) under test. Anotherapplication of interest is the process control of paper, i.e. mechanicalstrength, water content, etc.

With regard to flaw detection, it is well known to use high frequencyultrasound, such as that derived from a piezoelectric transducer, fordetecting small cracks and flaws during target inspection. It is knownto use a mode locked laser to generate ultrasonic waves at frequenciesas high as 100 MHz. It is also known that in addition to the detectionof small flaws, the mode-locked laser operation can generate very narrowband ultrasound (not necessarily high frequency).

This is exploited in the system 10 to substantially improve the SNR of ameasurement by reducing the electronics bandwidth. Particularly in themeasurement of thin targets (for example, under 0.1" for metals) theLamb modes of the target are dispersive, which implies that thethickness of the target affects the received pulse. This dispersivebehavior is problematic since it necessitates the prior knowledge of thethickness. However, this behavior disappears for thicker targets or forhigher frequency ultrasound. In other words, theory predicts that thewaves are not dispersive for high values of the product of frequency andthickness. Therefore, for very thin targets the high frequencyultrasound may be used to avoid the dispersive characteristics of thewaves.

It is pointed out that there are many ways of analyzing the receivedsignals. As was described previously, one technique is to first detectthe arrival of the wave (peak detection) and then from theTime-of-Flight (TOF) calculate the velocity by knowing the distance orthe path length. The determined elastic wave velocity correlates withmany properties of the target, such as temperature, metallurgicalstatus, etc. However, the velocity measurement is not the only suitablemethod for analyzing the signal.

In accordance with a further aspect of this invention any time varyingcharacteristic of the received elastic wave may be used to monitorvarious material properties, such as carbon content, case depth, grainsize, etc. Exemplary time varying characteristics include the frequencyof the received elastic wave, the phase of the received elastic wave,and an amplitude modulation of the envelope of the received elasticwave. Methods such as Fast Fourier Transformations, Wavelets, etc. areused in the system 10 to measure the time varying content, such asfrequency characteristics, of the received elastic wave so as tocorrelate the measured time varying content with one or more materialproperties of interest.

Having thus described the invention, it will be appreciated that anumber of modifications may be made to the disclosed embodiments. As anexample, although the trigger on demand technique has been previouslydescribed with respect to FIGS. 5, 6, 9 and 14, it should be realizedthat it is also within the scope of the invention to employ a mechanism,such as a piezoelectric actuator, to mechanically dither the referenceleg to provide an active stabilization. Furthermore, the activestabilization of the reference leg can be used in conjunction with thetrigger on demand technique.

Also by example, it is within the scope of this invention to eliminate athickness test altogether when determining a material's properties. Moreparticularly, when thick targets are being investigated (e.g., thickerthan 0.125") and bulk waves are being used, then there is a need forprior sided (reflected waves from opposite side) and double sided (onepass through target) configurations. In both of these cases the traveldistance of the elastic wave needs to be known in order to calculate thevelocity from the TOF measurements.

As is illustrated in the flow chart of FIG. 17, a method for determiningthe travel distance, without knowing the thickness a priori, is togenerate an elastic wave in the target and to then detect the arrival ofboth Longitudinal (L) and Shear (S) waves. For metals, the latter travelwith almost half the speed of the former. Therefore these waves arriveat different points in time and can thus be detected separately. Afterboth of the waves are received and detected (Block A), the ratio of thedetermined L and S TOFs are used to infer the ratio of the L and Svelocities (Block B). The travel path drops out of this calculation,since it is the same for both waves. Therefore, instead of correlatingthe velocity of each wave with the target properties, the ratio of the Land S velocities is correlated with a target property of interest (BlockC), thereby avoiding any thickness uncertainty.

It should be noted that this capability is an important feature of thesystem 10, which uses interferometric detection and can thereforemonitor both L and S waves. Conventional contact piezoelectrictransducers used in ultrasonic inspections lack this capability, andspecial transducers are used for each wave set.

Thus, while the invention has been particularly shown and described withrespect to preferred embodiments thereof, it will be understood by thoseskilled in the art that changes in form and details may be made thereinwithout departing from the scope and spirit of the invention. Further byexample, the use of a diode detection laser is not mandatory for thenovel signal processing techniques that were described previously, andneither is the use of a laser as the source of impulse energy to launchelastic waves within a sample.

What is claimed is:
 1. A system for determining a characteristic of atarget having a thickness suitable for supporting Lamb modes,comprising:means for launching an elastic wave within the target; aninterferometer for detecting a displacement of a surface of the targetin response to said launched elastic wave; means, responsive to saiddetected displacement, for determining a velocity of an elastic wavecorresponding to a symmetric (S_(o)) Lamb mode and a velocity of anelastic wave corresponding to an anti-symmetric (A_(o)) Lamb mode withinthe target; and means for correlating said determined S_(o) Lamb modeand said A_(o) Lamb mode velocities with a property of interest of thetarget.
 2. A system as set forth in claim 1 wherein a frequency of theelastic wave is made a function of the thickness of the target so as tosubstantially eliminate a dispersive characteristic of the elastic wave.3. A system as set forth in claim 1 wherein said property of interest istemperature.
 4. A system as set forth in claim 3 wherein the target iscomprised of a silicon wafer.